Display apparatus

ABSTRACT

A display apparatus includes an image forming device and a light guide. The light guide includes an incident portion and an emission portion including a plurality of mirrors. In the light guide, the total reflection surface is disposed to be inclined with respect to a virtual plane perpendicular to a visual axis, and the mirrors are disposed to be inclined with respect to the total reflection surface. The mirror angle is set such that the light guide angle, the mirror angle, the light angle, the limit incidence angle, and the refractive index of the light guide satisfy the predetermined relationship.

This is a Continuation of application Ser. No. 15/834,560 filed Dec. 7,2017, which in turn claims priority to Japanese Application No.2016-255073 filed Dec. 28, 2016. The entire disclosures of the priorapplications are hereby incorporated by reference herein in theirentirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a display apparatus.

2. Related Art

Recent years, as one of wearable information devices, an image displayapparatus in a type of being worn on the head of an observer such as ahead mount display is provided. In addition, an image display apparatuscapable of simultaneously and visually recognizing both an imagegenerated by a display element and an image of the outside of theobserver when the observer wears the image display apparatus, aso-called see-through type image display apparatus is known.

In JP-A-2012-42654, a display apparatus is disclosed, which includes animage forming device, a collimating optical system for collimating lightfrom the image forming device, a light guide optical device for guidingand emitting the light from the collimating optical system, and a movingdevice for relatively moving an optical axis of the image forming deviceand an optical axis of the collimating optical system and adjusting aconvergence angle. The light guide optical device includes a light guideplate and a plurality of mirrors provided inside the light guide plateso as to be inclined with respect to the plate surface.

In Japanese Patent No. 4,508,655, an optical device is disclosed, whichincludes a display source, a collimating lens, and a light-transmittingsubstrate. The light-transmitting substrate includes a first reflectionsurface that takes the light from the display source into the device anda second reflection surface that causes the light guided into the deviceto be emitted toward the eyes of the observer, and each of thelight-transmitting substrate is inclined with respect to the substratesurface.

As described above, the display apparatuses in JP-A-2012-42654 andJapanese Patent No. 4,508,655 include a plurality of mirrors orreflection surfaces for causing the light traveling inside the lightguide to be emitted toward the eyes of the observer. However, there is aproblem in that a display unevenness caused by the plurality of mirrorsor the reflection surfaces positioned in front of the eyes of theobserver is visually recognized.

SUMMARY

An advantage of some aspects of the embodiment is to provide a displayapparatus that can suppress the display unevenness from being visuallyrecognized.

The embodiment can be realized by in the following aspects. According toan aspect of the embodiment, a display apparatus includes an imageforming device and a light guide that guides an image light generated bythe image forming device. The light guide includes an incident portionthat causes the image light emitted from the image forming device to beincident on inside of the light guide, and an emission portion includinga plurality of mirrors that are provided parallel to each other insidethe light guide with an interval therebetween, and cause a part of theimage light guided inside the light guide to be reflected and causeanother part of the image light to be transmitted. The light guide has atotal reflection surface that guides the image light, and the totalreflection surface is disposed to be inclined with respect to a virtualplane perpendicular to a visual axis. Each of the plurality of mirrorsis disposed to be inclined with respect to the total reflection surface.When an inclination angle of the total reflection surface with respectto the virtual plane is defined as a light guide angle α, an inclinationangle of each of the plurality of mirrors with respect to the totalreflection surface is defined as a mirror angle β, an angle between alight beam incident on an eye of an observer and the visual axis isdefined as a light beam angle η, an incidence angle of the image lightwhen the reflectance of the image light becomes 0 with respect to eachof the plurality of mirrors is defined as a limit incidence angleθ_(th), and a refractive index of the light guide is n, the angle β ofeach of the plurality of mirrors is set so as to satisfy Expressions(1), (2), and (3) described below.β1<β≤β2  (1)β1=½×[sin⁻¹(1/n)+sin⁻¹(sin(η+α)/n)]  (2)β2=⅓×[θ_(th)+sin⁻¹(sin(η+α)/n)]  (3)The angle β of each of the plurality of mirrors is set so as to satisfythe above-described expressions (1), (2), and (3).

In the display apparatus according to the aspect, when the image lightis guided inside the light guide and is transmitted through a pluralityof mirrors, a part of the image light is reflected by the mirror and isemitted from the light guide toward the eyes of the observer. At thistime, the inventor have found that the error between the designed numberof mirrors through which the image light is to be originally transmittedin the design of the light guide and the number of mirrors through whichthe image light is actually transmitted influences the displayunevenness, and the display unevenness can be reduced as theabove-described error is reduced.

According to the display apparatus in an aspect of the embodiment, sincethe mirror angle β is set so as to satisfy the Expressions (1), (2), and(3), the error in the number of mirrors can be reduced, and thus, it ispossible to reduce the display unevenness. In addition, by satisfyingthe Expressions (1), (2), and (3) described above, both the loss of theimage light and the occurrence of a double image (ghost) are suppressed,and thus, a bright and clear image can be obtained.

In the display apparatus according to the aspect of the embodiment, thelight beam angle η may be equal to or larger than 10 degrees and equalto or smaller than 15 degrees, and the refractive index n may be equalto or larger than 1.4 and equal to or smaller than 1.8.

According to this configuration, it is possible to provide a highlypractical display apparatus.

In the display apparatus according to the aspect of the embodiment, thelight guide angle α may be equal to larger than 4 degrees.

An advantage of some aspects of the embodiment is that the displayunevenness can be more reliably reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiment will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a plan view of a display apparatus in an embodiment.

FIG. 2 is an enlarged view of an emission portion.

FIG. 3 is a diagram for describing a principle of occurrence of displayunevenness.

FIG. 4 is a graph illustrating a relationship between an incidence angleand a reflectance in a partially reflecting mirror.

FIG. 5 is a diagram illustrating a relationship between a mirror angle βand a standard deviation σ_(count) of an error of the number of mirrorsfor transmission.

FIG. 6 is a graph illustrating a relationship between a light guideangle α and a mirror angle β at a specific light beam angle η and arefractive index n.

FIG. 7 is a graph illustrating a relationship between the light guideangle α and the mirror angle β at a specific light beam angle η and arefractive index n.

FIG. 8 is a graph illustrating a relationship between the light guideangle α and the mirror angle β at a specific light beam angle η and arefractive index n.

FIG. 9 is a graph illustrating a relationship between the light guideangle α and the mirror angle β at a specific light beam angle η and arefractive index n.

FIG. 10 is a graph illustrating a relationship between the light guideangle α and the mirror angle β at a specific light beam angle η and arefractive index n.

FIG. 11 is a graph illustrating a relationship between the light guideangle α and the mirror angle β at a specific light beam angle η and arefractive index n.

FIG. 12 is a graph illustrating a relationship between an angle of viewand a conversion reflectance.

FIG. 13 is a graph illustrating a relationship between the light guideangle α and a standard deviation σ of the number of mirrors fortransmission.

FIG. 14 is a graph illustrating a simulation result of a light intensityprofile of an image at a pupil position in a specific diameter of alight receiver.

FIG. 15 is a graph illustrating a simulation result of a light intensityprofile of an image at the pupil position in a specific diameter of thelight receiver.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, an embodiment will be described using the drawings.

The display apparatus in the present embodiment is used as, for example,a head mount display.

FIG. 1 is a plan view of a display apparatus in an embodiment. FIG. 2 isan enlarged view of an emission portion.

In the following drawings, in order to make each configuration elementeasier to see, the scales of dimensions may be different from each otherdepending on the configuration elements.

Overall Configurations of the Light Guide Device and the DisplayApparatus

As illustrated in FIG. 1, the display apparatus 10 includes an imageforming device 11 and a light guide device 12. The display apparatus 10causes an observer to visually recognize the display image as a virtualimage and causes the observer to observe the external image in asee-through manner. In the display apparatus 10, a pair of image formingdevices 11 and light guide devices 12 are provided corresponding to theright eye and the left eye of the observer. Since the device for theright eye and the device for the left eye are symmetrical, only thedevice for the right eye is illustrated here and the device for the lefteye is omitted to be illustrated. The display apparatus 10 includes atemples (not illustrated) which is put on the ears of the observer, andhas an appearance like glasses as a whole.

The image forming device 11 includes a light source 14, a liquid crystalpanel 15, and a projection lens 16. The liquid crystal panel 15spatially modulates illumination light emitted from the light source 14to form a video image or other image light GL to be displayed. Theprojection lens 16 is a collimating lens which causes the image light GLemitted from each point on the liquid crystal panel 15 to be emitted assubstantially parallel light beams.

The projection lens 16 is formed with glass or plastic, and may beconfigured with plural pieces of lenses, not limited to one piece. Asthe projection lens 16, a spherical lens may be used, an asphericallens, a free-form surface lens, or the like may be used.

The light guide device 12 includes a light guide 18 and the light guide18 includes an incident portion 19 and an emission portion 21 having aplurality of mirrors 20. In a state in which the observer is wearing thedisplay apparatus 10, the incident portion 19 is provided on the sideclose to the ear, and the emission portion 21 is provided on the sideclose to the eye. The light guide 18 guides the image light GL insidethe light guide. The incident portion 19 causes the image light GLemitted from the image forming device 11 to be incident on the inside ofthe light guide 18. The emission portion 21 reflects the image light GLguided inside of the light guide 18 using a plurality of mirrors 20, anddeflects the image light to be emitted to the outside of the light guide18. The light guide device 12 emits the image light GL formed by theimage forming device 11 toward the eye EY of the observer as the virtualimage light, and transmits the external light EL forming the externalimage and leads the external image to the eye EY of the observer.

Here, a line of sight (optical axis) when the display apparatus 10 isseen from the front thereof by the eye EY of the observer at thedesigned pupil position of the display apparatus 10 is referred to as avisual axis AX. The angle formed by the light beam incident on the eyeEY of the observer with respect to the visual axis AX is defined as alight beam angle η.

The light guide 18 is configured with a flat light transmitting member.For example, the light guide 18 is molded with a resin material or aglass material having a high light transmittance. The light guide 18includes two total reflection surfaces 18 a which are parallel to eachother. In a state in which the observer wears the display apparatus 10on the head, among the two total reflection surfaces 18 a, a surfacepositioned at the observer side is referred to as an observer surface 18a 1 and a surface positioned at the external side is referred to as anexternal surface 18 a 2.

As illustrated in FIG. 2, in the light guide 18, the total reflectionsurface 18 a (the observer surface 18 a 1 and the external surface 18 a2) is disposed so as to be inclined with respect to a virtual plane KPperpendicular to the visual axis AX. Here, an inclination angle of thetotal reflection surface 18 a with respect to the virtual plane KP isdefined as a light guide angle α. Since the observer surface 18 a 1 andthe external surface 18 a 2 are parallel to each other, the angle formedby the observer surface 18 a 1 and the virtual plane KP and the angleformed by the external surface 18 a 2 and the virtual plane KP are equalto each other. However, in FIG. 2, the light guide angle α isillustrated as the angle formed by the observer surface 18 a 1 and thevirtual plane KP. In other words, the normal direction of the totalreflection surface 18 a of the light guide 18 is inclined by the angle αwith respect to the visual axis AX.

As illustrated in FIG. 1, the incident portion 19 is configured with amirror 23 provided inside the light guide 18. A reflection film made ofa metal having high reflectance such as aluminum is used as the mirror23. Alternatively, a dielectric multilayer film in which a plurality ofdielectric thin films having different refractive indexes arealternately laminated may be used as the mirror 23. The mirror 23 isprovided to be inclined with respect to the observer surface 18 a 1 ofthe light guide 18 in such a direction that an end portion 23 a of themirror 23 on the external surface 18 a 2 side inclines toward theemission portion 21 side. The inclination angle ω of the mirror 23 isappropriately set according to the positional relationship between theimage forming device 11 and the light guide 18.

The emission portion 21 includes a plurality of mirrors 20 providedinside the light guide 18 in parallel with each other at the sameinterval (pitch). Each of the plurality of mirrors 20 reflects a part ofthe image light GL guided in the inside of the light guide 18 andtransmits the other part of the image light GL. In this way, the mirror20 of the emission portion 21 functions as a so-called partiallyreflecting mirror. Like the mirror 23 of the incident portion 19, areflection film made of a metal having a high reflectance such asaluminum is used as the mirror 20. Alternatively, a dielectricmultilayer film may be used as the mirror 20.

Contrary to the mirror 23 of the incident portion 19, the mirror 20 ofthe emission portion 21 is inclined with respect to the observer surface18 a (total reflection surface 18 a 1) of the light guide 18 in such adirection that the end portion 20 a of the mirror 20 on the externalsurface 18 a 2 side collapses toward the incident portion 19 side.

As illustrated in FIG. 2, an inclination angle of the mirror 20 withrespect to the total reflection surface 18 a of the light guide 18 isdefined as a mirror angle β. In FIG. 2, the mirror angle β isillustrated as the inclination angle of the mirror 20 with respect tothe observer surface 18 a 1 of the light guide 18. However, it isassumed that 0°<β<90°. In addition, an emission angle of the light beamemitted from the observer surface 18 a 1 of the light guide 18 isdefined as γ.

The image light GL incident on the emission portion 21 is deflected atthe emission portion 21 so as to be extracted from the light guide 18,and is emitted from the observer surface 18 a 1. The image light GLemitted from the observer surface 18 a 1 is incident on the eye EY ofthe observer as virtual image light. By an image formed on the retina ofthe observer by the virtual image light, the observer can recognize thevirtual image by the image light GL.

Regarding the Principle of the Display Unevenness and Measures forImprovement

Here, the principle of occurrence of the display unevenness in thedisplay apparatus in the related art will be described with reference toFIG. 3.

The inventor have found that the number of mirrors through which thelight beam is transmitted is different depending on the angle of view ofthe light beam, and this fact becomes a cause of the display unevennessbeing visually recognized. In addition, even though the angles of viewof the light beams are the same, the number of mirrors for transmissionvaries depending on the mirror angle β.

Here, in order to evaluate the display unevenness, a parameter called anerror of the number of mirrors for transmission is considered. The“error of the number of mirrors for transmission” is a deviation of thenumber of mirrors through which the light beam actually transmitted withrespect to the number of mirrors through which the light beam to beoriginally transmitted at a predetermined angle of view. For example,the second mirror from the incident portion side needs to originallydeflect the light beam transmitted through one mirror in the precedingstage to the observer side, and the fifth mirror from the incidentportion side needs to deflect the light beam transmitted through fourmirrors in the previous stage. That is, the design of the light guidedevice is performed based on a point that the light beam is transmittedthrough one mirror once.

However, focusing on the light beam GL2 illustrated by a dashed line inFIG. 3, due to the relationship between the angle of view and the mirrorangle β, the light beam GL2 is transmitted twice through the secondmirror 202 at the positions indicated by reference numerals 20 b and 20c from the incident portion side. The number of transmissions throughthe mirror is counted as the transmission from the front side of thelight beam to the back side in the traveling direction of the lightbeam, and is not count as the transmission from the back side of thelight beam to the front side in the traveling direction. Therefore,regarding the light beam GL2, the third mirror 203 from the incidentportion side has to deflect the light beam after being transmitted twicein total through the mirrors 201 and 202 in the previous stage, butdeflects the light beam after being transmitted three times in totalthrough the mirrors 201 and 202.

On the other hand, focusing on the light beam GL3 illustrated by a solidline in FIG. 3, the light beam GL3 is transmitted only once through thesecond mirror 202 at the position indicated by a reference numeral 20 dfrom the incident portion side. Therefore, regarding the light beam GL3,the third mirror 203 from the incident portion side deflects the lightbeam after being transmitted twice in total through the mirrors 201 and202 in the previous stage, which is a transmission state as designed. Asa result, the intensity of the light beam GL2 lower than that of thelight beam GL3. Due to this reason, the intensity of the light beamsarriving at the observer at different angles becomes uneven, and thus,the display unevenness occurs on the whole image.

From the above consideration, if the error of the number of mirrors fortransmission can be made zero throughout all the display angles of view,it is possible to make the most desirable state in which there is nodisplay unevenness. Therefore, the standard deviation σ_(count) of theerror of the number of mirrors for transmission throughout all thedisplay angles of view is used as an evaluation index of the displayunevenness. As the standard deviation σ_(count) count decreases, thedisplay unevenness can be improved.

FIG. 5 is a diagram illustrating a relationship between a mirror angle βand a standard deviation σ_(count) of an error of the number of mirrorsfor transmission. In FIG. 5, the horizontal axis represents the mirrorangle β (degree) and the vertical axis represents the standard deviationσ_(count) (−).

As can be seen from FIG. 5, by increasing the mirror angle β, thestandard deviation σ_(count) can be reduced, and thus, it is possible toreduce the variations in the error of the number of mirrors fortransmission. That is, in order to improve the display unevenness, it ispreferable to increase the mirror angle β.

Regarding Limitations on Mirror Angle β

Incidentally, when setting the mirror angle β, there are two limitationsas follows.

One limitation is a fact that the light beam of the whole angles of viewhas to propagate inside the light guide 18 without loss. To this end,the light beam of the whole display angles of view needs to satisfy thetotal reflection condition. In order to satisfy the total reflectioncondition, the light beam needs to be incident on the total reflectionsurface 18 a of the light guide 18 with a large incidence angle, thatis, the light beam needs to be close to parallel to the total reflectionsurface 18 a. For that purpose, the mirror angle β has to be larger thana threshold value β1 of the mirror angle at which the light beamsatisfies the total reflection condition.

When the refractive index of the light guide 18 is n, the abovecondition is expressed by the following Expression (4).β>β1=½×[sin⁻¹(1/n)+sin⁻¹(sin(η+α)/n)]  (4)

Actually, the light beam deflected by the mirror 23 of the incidentportion 19 reaches the emission portion after propagating inside thelight guide 18, and is deflected by the mirror 20. Therefore, it may bedifficult to understand the idea of adjusting the mirror angle β of theemission portion 21 in such a manner that the light beam satisfies thetotal reflection condition with respect to the total reflection surface18 a of the light guide 18. However, the inventor obtained a findingthat, in a case of considering the mirror angle β of the emissionportion 21, in order for a light beam of a certain specific angle ofview to be incident on the eye EY of the observer, it is easy to study aconsideration of configuring the light guide 18 by tracing the path inthe direction opposite to the actual traveling path of the light beamfrom the eye EY of the observer in such a manner that the light guide 18satisfies a certain specific condition. The description above is basedon this finding.

Another limitation is a fact that the light beam having a largeincidence angle to the mirror 20 should not be reflected. In FIG. 3, oneof the reflected light beams from the mirror 20 is indicated by an arrowof a reference sign GL3 r, but when the light beam having the largeincidence angle to the mirror 20 is reflected from the mirror 20, theangle of the reflected light beam GL3 r slightly deviates from the angleof the transmitted light beam GL3 t. Due to this, there occurs anotherproblem that a double image (ghost) is generated.

In order to reduce the incidence angle to the mirror 20, the light beamneeds to be incident on the mirror 20 from a direction closer toperpendicular to the mirror 20. For that purpose, for example, as can beseen from the light beam GL3 incident on the first mirror from theincident portion side in FIG. 3, the mirror angle β has to be equal toor smaller than a specific threshold value β2.

The above condition is expressed by following Expression (5).

θ_(th) is a light beam incidence angle to the mirror 20 when thereflectance of the light beam in the mirror 20 is 0%, and is referred toas a limit incidence angle.β≤β2=⅓×[θ_(th)+sin⁻¹(sin(η+α)/n)]  (5)

The threshold value β1 is uniquely determined by the refractive index nof the light guide 18 and the light beam incidence angle with respect tothe light guide 18. On the other hand, the threshold value 132 dependson the reflection characteristics of the mirror 20. FIG. 4 illustratesan example of the incidence angle—reflectance characteristics of themirror used for this type of light guide device. For example, in a casewhere the mirror having the characteristics illustrated in FIG. 4 isused, in order to make the reflectance 0%, the limit incidence angleθ_(th) is set to equal to or lower than 80°.

In addition, the emission angle γ (refer to FIG. 2) of the light beamfrom the total reflection surface 18 a (observer surface 18 a 1) of thelight guide 18 is determined by the light guide angle α and the lightbeam angle η. That is, γ=α+η.

In a case where the light beam angle η is 10 degrees or 15 degrees andthe refractive index n of the light guide 18 is 1.41 which is therefractive index of the silicone resin, 1.52 which is the refractiveindex of BK7 (glass material), or 1.73 which is the refractive index ofepisulphide (resin material), graphs indicating the light guide angledependences of each mirror angle are illustrated in FIG. 6 to FIG. 11.

FIG. 6 is a graph in a case where η=15 degrees and n=1.41. FIG. 7 is agraph in the case where η=15 degrees and n=1.52. FIG. 8 is a graph in acase where η=15 degrees and n=1.73. FIG. 9 is a graph in a case whereη=10 degrees and n=1.41. FIG. 10 is a graph in a case where η=10 degreesand n=1.52. FIG. 11 is a graph in a case where η=10 degrees and n=1.73.

In FIG. 6 to FIG. 11, the horizontal axis of the graph represents thelight guide angle α (degree) and the vertical axis represents the mirrorangle β (degree).

In FIG. 6 to FIG. 11, dashed lines of signs A6 to A11 are graphs(Expression (4)) illustrating the relationship between β1 and α, andsolid lines of signs B6 to B11 are graphs (Expression (5)) illustratingthe relationship between β2 and α. Therefore, in each figure, regions onthe upper side of the graphs of signs A6 to A11 and lower side of thegraphs of signs B6 to B11 (hatched region) are the regions satisfyingβ1<β≤β2, and the value of the light guide angle α can only be obtainedin the range corresponding to the region described above. For example,in the case where η=15° and n=1.41 (FIG. 6), it is necessary to setα<20°. In a case where η=15° and n=1.52 (FIG. 7), it is necessary to setα<45.

In a case where the display apparatus 10 is used as the head mountdisplay, since the light guide 18 needs to be disposed along the frontface of the face portion, the angle α=45° is too large. From thisviewpoint, the light guide angle α is preferably approximately 5° to25°. On the other hand, in a case where the display apparatus 10 is usedas a head-up display, the light guide angle α can be selected at a pointup to the intersection of the two graphs.

Here, the inventor examined the actual occurrence of the displayunevenness when changing the light guide angle α.

As described above, as the display unevenness appears due to thevariations of the number of mirrors for transmission and as the mirrorangle β increases, the display unevenness is reduced. Therefore, themirror angle β is adjusted so as to be incident with the upper limit ofthe mirror angle β at the limit incidence angle θ_(th)=80°. In addition,the reflectance of the mirror 20 is adjusted such that the reflectanceincreases from the side close to the incident portion 19 to the side farfrom the incident portion 19. For example, in order to make thereflectance at all the angles of view to be 10%, the reflectance of thefirst mirror from the incident portion side may be set to 10%, thereflectance of the second mirror may be set to 11.1% (=0.1/(1−0.1)), andthe reflectance of the third mirror may be set to 12.5%(=0.1/(1−2×0.1)).

FIG. 12 is a graph illustrating an integrated reflectance which is amultiplication of the number of mirrors for transmission and thetransmittance and the reflectance throughout the angles of view ±15degrees under the condition of the reflectance described above and therefractive index n=1.52. In FIG. 12, the horizontal axis represents theangle of view (degree) and the vertical axis represents the integratedreflectance (−). The graph indicated by the dashed line of sign C12illustrates a case where the light guide angle α is 0° (in a case wherethe light guide is not inclined), the graph indicated by two-dottedchain line of sign A12 illustrates a case where the light guide angle αis 10°, and the graph indicated by a solid line of sign B12 illustratesa case where the light guide angle α is 14°.

In FIG. 12, since the reflectance of each mirror is set such that thereflectance becomes 10% through all the angles of view, originally it isdesirable that the integrated reflectance be constant at a value of 0.1.In a case where the light guide angle α is 10° (A12) and in a case wherethe light guide angle α is 14° (B12), It is found that the variationfrom the integrated reflectance 0.1 becomes small compared to the casewhere the light guide angle α is 0° (C12).

FIG. 13 is a graph illustrating a relationship between the light guideangle α in FIG. 12 and a standard deviation σ of integrated reflectance.In FIG. 13, the horizontal axis represents the light guide angle α(degree), and the vertical axis represents the standard deviation σ(a.u.).

As illustrated in FIG. 13, as the light guide plate angle α becomeslarger than 0 degree, the standard deviation σ tends to decrease and itcan be understood that the display unevenness decreases. Particularly,when the light guide plate angle α becomes equal to or larger than 4degrees, it is found that the slope of the graph becomes larger than theslope of the graph when the light guide plate angle α is smaller than 4degrees. Therefore, in this example, it is found that the effect ofreducing the display unevenness can be clearly obtained when the lightguide plate angle α is set to equal to or larger than 4 degrees.

In addition, the inventor obtained a light intensity distribution of theimage at the pupil position when the reflectance of the mirror is 10%.

FIG. 14 is a graph illustrating the light intensity distribution of theimage at the pupil position when the diameter of the light receiver is0.5 mm.

FIG. 15 is a graph illustrating the light intensity distribution of theimage at the pupil position when the diameter of the light receiver is 3mm.

In FIG. 14 and FIG. 15, the horizontal axis of the graph represents theangle of view (degree), and the vertical axis of the graph representsthe light intensity (a.u.). The solid line graphs of the signs C14 andC15 illustrate the case where the light guide angle α is 0° (when thelight guide is not inclined), and the dashed line graphs of the signsA14 and A15 illustrate the case where the light guide angle α is 15.4°.

As illustrated in FIG. 14, when the diameter of the light receiver isreduced, the change in light intensity can be clearly known. On theother hand, since generally a diameter of the human pupil isapproximately 3 mm, as illustrated in FIG. 15, if the diameter of thelight receiver is set to 3 mm, it is possible to estimate how the imageis recognized in the eyes of the observer. As can be seen from bothgraphs, in a case where the angle of the light guide is inclined, thelight intensity distribution can be kept small throughout all the anglesof view. As a result, it is possible to reduce the display unevenness.

As described above, in the display apparatus 10 in the presentembodiment, by inclining the light guide 18 with respect to the virtualplane KP perpendicular to the visual axis AX, and appropriately settingthe mirror angle β, it is possible to reduce the error of number ofmirrors for transmission when the image light GL is transmitted througha plurality of mirrors 20. As a result, the display unevenness can bereduced. Particularly, since the mirror angle β is set so as to satisfyEquations (4) and (5) above, both the loss of the image light and theoccurrence of the double image (ghost) can be suppressed, and thus, abright and clear image can be obtained.

In addition, by inclining the light guide 18, the light guide 18 can bedisposed along the front face of the face, and the degree of freedom indesigning the display apparatus 10 is improved.

In addition, by setting the light beam angle η equal to or larger than10 degrees and equal to or smaller than 15 degrees, and setting therefractive index n of the light guide 18 equal to or larger than 1.4 andequal to or smaller than 1.8, it is possible to provide a highlypractical display apparatus 10. Furthermore, by setting the light guideangle α equal to or higher than 4 degrees, it is possible to morereliably reduce the display unevenness.

The technical scope of the invention is not limited to the embodimentdescribed above, and various modifications can be made without departingfrom the spirit of the embodiment.

For example, the specific configuration of each part such as the number,shape, material, or the like of the respective configuration elementsconfiguring the display apparatus is not limited to the embodimentdescribed above, and can be appropriately changed. For example, as theimage forming device, in addition to the above liquid crystal displayapparatus, an organic EL device, a combination of a laser light sourceand a MEMS scanner, or the like may be used.

The entire disclosure of Japanese Patent Application No. 2016-255073,filed Dec. 28, 2016 is expressly incorporated by reference herein.

What is claimed is:
 1. A display apparatus comprising: an image forming device that generates image light; and a light guide that guides the image light, wherein the light guide includes a first surface, a first mirror, and a second mirror, the image forming device causes the image light to enter an inside of the light guide from the first surface, the first mirror (i) causes a part of the image light guided inside the light guide to be reflected, and (ii) causes another part of the image light to be transmitted, the second mirror (i) causes a part of the image light guided inside the light guide to be reflected, and (ii) causes another part of the image light to be transmitted, the first mirror is disposed to be inclined with respect to the first surface, the second mirror is disposed to be inclined with respect to the first surface, in a plan view of the display apparatus, the first surface is disposed to be inclined with respect to a virtual plane perpendicular to a visual axis, and when (i) an inclination angle of the first surface with respect to the virtual plane is defined as a light guide angle α, (ii) an inclination angle of each of the first mirror and the second mirror with respect to the first surface is defined as a mirror angle β, (iii) an angle between a light beam incident on an eye of an observer and the visual axis is defined as a light beam angle η, the light beam being a light beam that forms an angle of view among the image light, (iv) an incidence angle of the image light when the reflectance of the image light becomes 0 with respect to each of the first mirror and the second mirror is defined as a limit incidence angle θ_(th), and (v) a refractive index of the light guide is n, the angle β of each of the first mirror and the second mirror is set so as to satisfy Expressions (1), (2), and (3) described below: β1<β≤β2  (1) β1=½×[sin⁻¹(1/n)+sin⁻¹(η+α)/n)]  (2) β2=1/3×[θ_(th)+sin⁻¹(sin(η+α)/n))]  (3) in the plan view, the light guide angle α at which the first surface is inclined with respect to the virtual plane is larger than 0 degrees.
 2. The display apparatus according to claim 1, wherein expressions (4) and (5) below are satisfied: 10 degrees the light beam angle η≤15 degrees  (4), and 1.4 the refractive index n≤1.8  (5).
 3. The display apparatus according to claim 1, wherein the light guide angle α is equal to or larger than 4 degrees.
 4. The display apparatus according to claim 1, wherein the light guide includes a second surface, the second surface is opposite the first surface in the plan view, and the light guide guides the image light by the first surface and the second surface.
 5. The display apparatus according to claim 1, wherein the light guide includes an incident portion including a part of the first surface, and the image forming device causes the image light to enter the inside of the light guide from a part of the first surface included in the incident portion.
 6. The display apparatus according to claim 5, wherein the incident portion includes a mirror, the mirror of the incident portion is disposed to be inclined with respect to each of the first surface and the virtual plane, an inclination angle of the mirror of the incident portion with respect to the first surface is defined as an angle ω, an inclination angle of the mirror of the incident portion with respect to the virtual plane is defined as an angle A, and the inclination angle A of the mirror of the incident portion is set so as to satisfy an Expression A=ω+α. 